The long-term objective of our research is to understand how olfactory information is processed in the mammalian brain. To address this question, we study the properties of neurons in slices of the rat olfactory bulb-the first site in the brain where olfactory information is processed. While fast synaptic transmission in the brain is typically mediated by neurotransmitters released from axon nerve endings, dendritic transmitter release is thought to play an important role in the olfactory bulb. Glutamate released from principal mitral cell dendrites excites the dendrites of local interneurons, granule cells, which in turn release gamma-aminobutyric acid (GABA) back onto mitral dendrites. This dendrodendritic circuit underlies self- and lateral inhibition of mitral cells and has been suggested to play a critical role in odor discrimination and resolution in the olfactory bulb. The major goal of this research proposal is to elucidate the synaptic mechanisms governing dendrodendritic inhibition between mitral and granule cells in the olfactory bulb. The specific aims focus on characterizing the fundamental properties of transmission between the two cell types, as well as the role of GABAB receptors and activity-dependent plasticity in modulating dendritic signaling. Mitral and granule neurons are visualized with infrared differential interference optics (IR-DIC) and studied using patch-clamp recording techniques. Specific Aim I proposes to establish the basic quantal mechanisms governing the strength of dendritic transmission between synaptically coupled pairs of mitral and granule cells. Specific Aim 2 proposes to investigate the role of GABAB receptors in dendrodendritic inhibition. We hypothesize that metabotropic GABAB receptors play an important role in modulating dendritic interactions between mitral and granule cells. Specific Aim 3 proposes to investigate synaptic plasticity at dendrodendritic synapses. We hypothesize that excitatory dendritic inputs to granule cells express activity-dependent long-term potentiation and depression. These experiments will provide new insight into the synaptic mechanisms underlying dendrodendritic transmission, which is of critical importance in the processing of olfactory information in the brain.